A different kind of 'invisibility cloak' can serve as heat shield

This schematic shows that the object in the center of a thermal invisibility cloak stays cold while the heat diffuses elsewheree. The source of the heat is on the left side, maintaining a temperature of 100 degrees Celsius (212 degrees Fahrenheit).

By now, you've heard of "invisibility cloaks" that can hide objects from prying eyes, or military scouts, or sonar scopes — but how about a cloaking device that can keep your computer circuits cool?

That's just the kind of thermal cloaking device that French researchers are proposing in the journal Optics Express, and it might not be too long before it becomes a reality.

"We expect to have the first prototype ready in a few months, since as usual there are a few fabrication constraints which need to be fixed, but nothing really serious," lead researcher Sebastien Guenneau told me in an email. "All seems to be under control."

Most of the invisibility cloaks under development work by using metamaterials to bend light waves or sound waves around a shielded object, making the object undetectable in those wavelengths. Guenneau, who is affiliated with the University of Aix-Marseille and France's Centre National de la Recherche Scientifique, decided to work with his CNRS colleagues to adapt the wave-bending approach to thermal diffusion.

"Our key goal with this research was to control the way heat diffuses in a manner similar to those that have already been achieved for waves ... by using the tools of transformation optics," Guenneau said in a news release issued by the Optical Society, which publishes the open-access Optics Express.

Instead of controlling wave propagation, the thermal cloak would control the flow of heat. "The mathematics and the physics at play are much different," Guenneau explained. "For instance, a wave can travel long distances with little attenuation, whereas temperature usually diffuses over smaller distances."

The basic design of the thermal invisibility shield is similar, however: Rings of specially shaped material guide the heat flow along the desired path.

"We can design a cloak so that heat diffuses around an invisibility region, which is then protected from heat. Or we can force heat to concentrate in a small volume, which will then heat up very rapidly," Guenneau said.

"The flow of heat follows the direction of highest diffusivity, which in our case is around the invisibility zone," he wrote. "Earlier thermal protections require you to basically surround the region to protect with a coating with low diffusivity (e.g., air or polymer, just like your double-glazed windows). To use an analogy with optics, it's just like putting Harry Potter in a box and saying, 'Look, you cannot see Harry anymore, he has been made invisible.' Our approach is to really make Harry invisible, so we should not see the box either."

So what's this cloak going to be made of? In the paper, Guenneau and his colleagues say the materials that go into a concentric multilayered cloak could range from PVC-type polymers to metals such as silver and gold. Production of the prototype cloak is currently under way at the University of Lille, Guenneau said, "but I cannot reveal exactly what it is made of at this stage."

In the next few months, we should be hearing a lot more about the thermal invisibility cloak from Guenneau and his French colleagues ... provided they don't disappear.